1. Field of the Invention
The present invention relates to a system for measuring timing relationship between signals such as a time difference between two signals.
2. Related Background Art
In measuring a time difference between two signals, a device for displaying a signal waveform such as an oscilloscope (including a storage oscilloscope) is usually used. This is briefly explained below. Signal waveforms are displayed on a display screen of the device and a difference on time axis on the display screen is measured to detect a timing relationship between the signal forms. For example, for signal waveforms having peaks, peak positions of the waveforms are observed to detect the time difference on the time axis.
In an IC tester, a fast rise pulse signal is applied to each pin and a delay between input and output of the signal is measured. If there is a difference between distances from a pulse signal source to the respective pins, the pulse signals at the respective pins rise with a time difference (Assuming that a velocity of an electrical signal is equal to a light velocity, there is a difference of 10 ps for 3 mm). Accordingly, it is necessary to calibrate the time difference between the pulse signals and it is usually done in the following manner. Near the pin, the pulse signal is applied to a comparator to detect a time difference between crossings of a threshold level, and the timing from the pulse signal source is adjusted to make the time difference zero. After the calibration, the same comparator is used to measure the input/output timing difference by the difference between crossings of the threshold level.
When the device for displaying the signal waveform is used, if a noise is included in the signal, a portion to be compared is made vague by the noise and it is not possible to precisely measure the time difference. Further, because of visual measurement, a visual measurement error is necessarily included. In addition, if the waveform is distorted, it is hard to specify the portion to be compared, and measurement result differs from person to person.
Similar problems are encountered when the time difference is measured by a threshold level. If the noise is included, the threshold is misdetected due to the noise, and if the waveform is distorted, the time difference between crossings of the threshold level does not always correspond to a time difference between signals.
Thus, there has been a limit in the precise measurement of the time difference between the two signals.